Non-Defining  Relative Clauses explanation:

 http://www.usingenglish.com/glossary/non-defining-relative-clause.html

Get some practice at :

http://www.usingenglish.com/quizzes/51.html

http://www.usingenglish.com/quizzes/234.html

http://www.usingenglish.com/quizzes/373.html